The present invention relates to a method for forming an align key pattern in a semiconductor device and, more particularly, to a method for forming an align key pattern in a relatively wide align key pattern area on a semiconductor substrate.
As the level of integration in semiconductor devices has increased, device sizes have become smaller. Along with the trend, device isolation structures for electrical isolation between adjacent devices have also become smaller. Device isolation structures using a widely-used local oxidation of silicon (LOCOS) method currently reach the limits of their effectiveness, and thus they are difficult to adopt for applications to more highly integrated semiconductor devices. Accordingly, new alternatives are required to overcome the limitations of the LOCOS structures for device isolation and achieve device isolation structures needed for high integration. One of the alternatives is shallow trench isolation (STI) structures.
For STI structures, trenches are formed into a semiconductor substrate. Thereafter, the trenches are filled with an insulating material, like an oxide film, to a thickness sufficient to bury the trenches. Subsequently, the insulating materials etched back by a chemical mechanical polishing (CMP) method until the surface of an adjacent active region is exposed, thereby planarizing the whole surface of the semiconductor substrate having the trenches formed thereon. Thus, the device isolation structure is completed. The STI method is free of bird's beaks and has an advantage over the LOCOS structures with respect to minimization of isolation spacing. It possesses, however, a distinct drawback that an align key pattern necessary for subsequent photolithography steps is not obtained, since no step is created on the surface of the semiconductor substrate in the STI structure. Conventionally, when a laser is projected from an aligner of a stepper and reflected on an align key pattern formed on the semiconductor substrate, an interference pattern formed due to the irregularities of the align key pattern. The interference pattern is recognized in a detector, the direction and position of the semiconductor substrate are detected, and then the semiconductor substrate and equipment are adjusted in accordance with the detected direction and position of the semiconductor substrate, thereby performing alignment. However, in the STI, a device isolation oxide film is formed by etching back through using the CMP method or the like, thus providing a planar semiconductor substrate without any step between the device formation area and the device isolation area.
When an opaque film like a tungsten silicide, used as a gate electrode material, is formed on such a planar surface, an interference pattern due to reflection will not be formed. The alignment of the photolithographic equipment, therefore, is practically difficult to perform.
A method for forming align key patterns in a LOCOS and a STI structure using conventional technology will now be described in detail, referring to the attached drawings.
FIGS. 1A and 1B are plan views of the align key patterns in the LOCOS and STI structures using conventional technology. FIGS. 2A and 2B are vertical section views of the align key patterns of the LOCOS and STI structures using conventional technology.
FIG. 1A is a plan view of the align key pattern of the LOCOS structure. Reference numeral 12 denotes active regions and reference numeral 10 denotes field regions. As shown, in the field oxide film having a LOCOS structure, a distinct align key pattern is produced due to steps formed on the semiconductor substrate. As shown in FIG. 2A, which is a schematic cross-section of the LOCOS structure, steps of lower device isolation oxide films 20 formed on the surface of the semiconductor substrate are transcribed to an opaque film 22, e.g. a tungsten silicide which is formed on the whole surface of oxide films 20, and to a sensitized film 24, thereby forming a distinct align key pattern as shown in FIG. 1A.
FIG. 1B is a plan view of the align key pattern of the STI structure. Reference numeral 14 denotes active regions and reference numeral 16 denotes field regions. As shown in FIG. 2B, in the device isolation structure having a STI structure, no step is formed between the field region and the active region. Moreover, since an opaque material such as a tungsten silicide is formed on the whole surface of the semiconductor substrate having the STI structure, the steps on the lower layer cannot be sensed, thereby making a distinct formation of an align key pattern, required for photolithography, impossible.
FIG. 2A is a vertical cross-sectional view of the align key pattern of the LOCOS structure. On semiconductor substrate 18, is formed field oxide films 20 and then tungsten silicide 22. A photoresist 24 is covered on the whole surface of tungsten silicide 22. The LOCOS structure permits a distinct formation of an align key pattern due the steps formed by field oxide films 20. On the other hand, the LOCOS structure has a in that a bird's beak formed at the edge of the field oxide film contributes to the reduction in the area of an adjacent active region.
FIG. 2B is a vertical section view of the align key pattern of the STI structure. Trenches 26 are formed into semiconductor substrate 25, and then field oxide films 27 are formed on the whole surface of semiconductor substrate 25 thereby burying or filling trenches 26. Thereafter, the whole surface of the resultant substrate is etched back until the interface of an active region 28 is exposed, which results in the planarization of the whole surface of the substrate. On the whole surface of the planarized substrate, is formed tungsten silicide 29. A photoresist 30 is covered on the whole surface of tungsten silicide 29. Field oxide films 27 of the STI structure do not produce steps in relation to active region 28, and an opaque material like the tungsten silicide is formed on the whole surface of field oxide films 20, thereby making it difficult to obtain a distinct align key pattern, unlike in the LOCOS structure.
As described above, it is difficult to form an align key pattern on a semiconductor substrate of a STI structure by using the conventional align key pattern formation method, since no steps are created between the field region and the active region.